Germanium (Ge) is a key semiconductor material in photonic and optoelectronic applications due to its superior light absorption properties in the near-infrared (NIR) wavelength range. Wavelengths beyond 1.55 μm are especially important because the wavelength range for dense wavelength division multiplexing (DWDM) technology expands to the L-band (i.e., 1561˜1620 nm). However, Ge photodetectors typically suffer from low responsivity at wavelengths longer than 1.55 μm due to the abrupt drop in their absorption coefficient beginning at a wavelength of ˜1.5 μm.
Ge nanomembranes (Ge NM), which may have, for example, thicknesses ranging from several tens of nanometers to hundreds of nanometers, have been used in optoelectronic applications. For example, Ge-based infrared optical gain media and flexible Ge photodetectors have been demonstrated. Although a thicker (i.e., bulk) Ge layer has the advantage of better light absorption, a thinner Ge layer (e.g., Ge NM) has more flexibility for device fabrication. Regardless of the thickness, the light absorption coefficient for Ge (both bulk Ge and Ge NM) decreases rapidly at wavelengths longer than 1.5 μm. Therefore, it is desirable to enhance the light absorption coefficient of Ge so that the total light absorption can be enhanced sufficiently to enable its use in Ge-based optical sensor applications for both long haul and on-chip optical communications.